1. Field of the Invention
The present invention relates to a filter device including a plurality of elastic wave resonators provided on a piezoelectric substrate, and particularly relates to a filter device and a duplexer including a longitudinally coupled resonator-type elastic wave filter section.
2. Description of the Related Art
Longitudinally coupled resonator-type filter devices have thus far been widely used as reception filters of duplexers in cellular phones. There is strong demand for the miniaturization of longitudinally coupled resonator-type filter devices.
In Japanese Unexamined Patent Application Publication No. 2008-118277, first and second longitudinally coupled resonator-type filter sections, which are each formed by cascade-connecting two longitudinally coupled resonator-type elastic wave filter units, are connected in parallel. In other words, a longitudinally coupled resonator-type elastic wave filter device having a two-stage, four-element configuration is disclosed. In Japanese Unexamined Patent Application Publication No. 2008-118277, ground terminals of adjacent interdigital transducer electrodes in the first-stage longitudinally coupled resonator-type elastic wave filter units of the first and second longitudinally coupled resonator-type elastic wave filter sections are electrically connected by wiring lines. Accordingly, the wiring lines function as virtual ground wiring lines, where the wiring lines are not connected to a ground potential. Accordingly, it is not necessary to connect the virtual ground wiring lines to a ground potential. This makes it possible to achieve miniaturization.
In the case where the virtual ground wiring lines disclosed in Japanese Unexamined Patent Application Publication No. 2008-118277 are used, terminals, wiring lines, and the like for connecting to a ground potential can be omitted. Accordingly, a higher level of miniaturization can be achieved.
However, in a balanced elastic wave filter device, there has been a problem in that when a ground wiring line is used, large ripples appear in characteristics such as an amplitude difference, a phase difference, and so on in pairs of balanced terminals. Meanwhile, in an unbalanced elastic wave filter device, there has been a problem in that large ripples appear within a pass band when virtual ground wiring lines are used. Accordingly, virtual ground wiring lines have thus far not been used.